1 The Dome An Unexpectedly Simple Failure of Determinism John D. Norton Center for Philosophy of...

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The DomeAn Unexpectedly Simple Failure of Determinism

John D. NortonCenter for Philosophy of Science and

Department of History and Philosophy of Science

University of Pittsburgh

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This talk…

1. The Indeterminism

2. What about Newton’s First Law?

3. What’s New?

4. What’s Wrong?

4a. Incomplete Formulation of Newton’s Laws?

4b. “Unphysical”

4c. Inadmissible Idealizations

5. When Determinism is Exceptional and Generic

6. Conclusion

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3. What’s New?

4. What’s Wrong?





6. Conclusion

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The Arrangement

The mass experiences an outward directed force fieldF = (d/dr) potential energy = (d/dr) gh = r1/2.

The motion of the mass is governed by Newton’s “F=ma”:

d2r/dt2 = r1/2.

A unit mass sits at the apex of a dome over which it can slide frictionless. The dome is symmetrical about the origin r=0 of radial coordinates inscribed on its surface. Its shape is given by the (negative) height function h(r) = (2/3g)r3/2.

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Possible motions: None

r(t) = 0solves Newton’s equation of motion since

d2r/dt2 = d2(0)/dt2 = 0 = r1/2.

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Possible motions: Spontaneous Acceleration

For t≤T, d2r/dt2 = d2(0)/dt2 = 0 = r1/2.

For t≥T

d2r/dt2 = (d2 /dt2) (1/144)(t–T)4

= 4 x 3 x (1/144) (t–T)2

= (1/12) (t–T)2

= [(1/144)(t–T)4]1/2 = r 1/2

The mass remains at rest until some arbitrary time T, whereupon it accelerates in some arbitrary direction.

r(t) = 0, for t≤T and

r(t) = (1/144)(t–T)4, for t≥Tsolves Newton’s equation of motion

d2r/dt2 = r1/2.

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The computation again

For t≤T, d2r/dt2 = d2(0)/dt2 = 0 = r1/2.

For t≥T

d2r/dt2 = (d2 /dt2) (1/144)(t–T)4

= 4 x 3 x (1/144) (t–T)2

= (1/12) (t–T)2

= [(1/144)(t–T)4]1/2 = r 1/2

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Without Calculus

Imagine the mass projected from the edge.

Close…

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Without Calculus


Closer…

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Without Calculus

BUT there is a loophole. Spontaneous motion fails for a hemispherical dome. How can the thought experiment fail in that case?

Now consider the time reversal of this process.

Spontaneous motion!


BINGO!

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3. What’s New?

4. What’s Wrong?





6. Conclusion

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At t=T, there is no net force on the mass and its acceleration vanishes.

Newton’s First law

“Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.”

There is no net external force at r=0. So how can the mass accelerate at r=0?

The law is respected.

The acceleration a(t) is:

For t<T, a(t) = 0

For t=T, a(t) = 0

For t > 0, a(t) = (1/12) (t–T)2

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There has to be a cause that sets the mass into motion at t=T! No First Cause?

There is no “first moment of motion” at which the first cause would act.

t=T is the last moment at which the mass is at rest.

There is no first moment at which the mass moves.

For any t>T, there is always an earlier moment at which the mass is moving.

And why must every process have a first cause?

“… unless it is compelled to change…”

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Newton’s Original Formulation of his Laws…

…is adapted to the polygonal trajectories he analyzed in Principia

Law 1. Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.

Law 2. The change of motion is proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.

For these trajectories, changes of motion always have first causes, the impulsive forces that coincide with the kinks.

First law applies to inertial segments.

Second law “change of motion” applies to the impulsive forces that create kinks.

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A Modern Formulation of Newton’s Laws…

Law 2. F = ma The first law is logically entailed by the second.

For these trajectories, periods in which motion changes may have no first instant and thus no instant at which the first cause can act.

…is adapted directly to motion governed by continuous forces.

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Newton’s formulation is compatible with continuously varying forces if:

Bruce Pourciau (2006) “Newton’s Interpretation of Newton’s Second Law,” Archive for History of Exact Sciences, 60, pp. 157-207.

“Change of motion” =

change of position of the body that accumulates over some non-zero time

“Motive force impressed”

= …

Change of position in non-zero time interval t=T to t=T+t is motion fromr=0 to r=(1/44)(t)4.

Motive force impressed is the accumulated effect of the instantaneous forces acting on the mass in the spatial interval0<r<(1/44)(t)4.

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3. What’s New?



3. What’s New?

4. What’s Wrong?





6. Conclusion

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Varieties of Newtonian Indeterminism

Space invader,

supertaskindeterminism

Individual components are locally well-behaved.Each only changes its motion if struck, pushed or pulled.

Indeterminism arises from assembling individually well-behaved parts.

Dome indeterminism

The mass changes it motion locally without any change in its surroundings.

Indeterministic process cannot be decomposed into locally well-behaved parts.

No way to “pop the hood” and find an engine with well-behaved parts

behind the dome.

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4. What’s Wrong?



3. What’s New?

4. What’s Wrong?





6. Conclusion

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The dome is not really licit in Newtonian theory because…

…it uses an incomplete formulation of Newton’s law.

…it uses inadmissible idealizations.

…it is “unphysical.”

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3. What’s New?

4. What’s Wrong?





6. Conclusion

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Incomplete Formulation of Newton’s Laws?

Intent of Newtonians was that their theory is deterministic.

Canonical examples of Newtonian systems are all deterministic.

The intent of the theorists does not fix the content of the theory

The theory is not strong enough to enforce determinism in all systems.

Bodies at rest were presumed to stay put!

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Just which fourth law are we to add to Newton’s three laws?

Complications

Merely stipulating determinism is too weak. Which of the many indeterministic motions in the Right One?

Stronger modifications may rule out good behaviors; or be too narrowly tailored to one example.

What is Newtonian theory? Not decided by experiment; theory is false.

Not decided as theorem of bigger system.

Best? Declared by agreement among physicists?

The community… …may declare a new four-law formulation of Newtonian theory.

…may NOT decree that the standard three law formulation never was a proper formulation.

if so

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3. What’s New?

4. What’s Wrong?





6. Conclusion

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“Unphysical”

Newtonian theory is approximately true of ordinary things.

The dome is not exploring the properties of the actual world.

The dome is exploring the properties of a false theory; we should expect that exploration to reveal behaviors not realized in the world.

Ordinary objects do not launch themselves into motion without being pushed.

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Three notions of “unphysical”

Notion: Unphysical as gauge (overdescription).

Unphysical = cannot obtain in the real world.

Example: Origin of coordinate system in Euclidean space.

Remedy: Do nothing, just remember it’s unphysical.

Relevance:NOT notion at issue, since indeterministic motions not due to arbitrary descriptive conveniences.

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2 Three notions of “unphysical”

Notion: Unphysical as false.Unphysical predictions are just wildly off what we see in the world.


Example: “Ultraviolet catastrophe”of classical electrodynamics.

Remedy: Reject theory as falseat least in this domain.

Relevance:Does not license excision of

dome, but full rejection of the theory.

Probably NOT notion since the dome cannot be realized in actual world.

The theory predicts infinite energy densities; we see finite energy densities.

Atomic nature of matter precludes building exact shape of dome.

Quantum theory prohibits placing a mass exactly at rest exactly at the apex of the dome.

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Eliminate asunphysical since the

surface is impenetrable

Three notions of “unphysical”

Notion: Unphysical as underdescription.Spurious solutions admitted by incompleteness in theory.


Example: Bouncing ball

Remedy: Excise spurious solution by hand.

Energy conservation entailsu2=v2

which has solutionsu=v and u=-v

Relevance:Closest to notion sought since it

permits excision of consequences.

BUT inapplicable, since we have no independent access to the target system.

NO basis to sort spurious from proper solutions.

Dome exists solely as idealization of theory.

Atomic nature of matter precludes building exact shape of dome.

Quantum theory prohibits placing a mass exactly at rest exactly at the apex of the dome.

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3. What’s New?

4. What’s Wrong?





6. Conclusion

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Table with sharp edge:

curvature diverges, and

tangent undefined.

Inadmissible Idealizations

The dome uses many idealizations whose denial would block the indeterminism.

The idealizations of the dome are no worse than those routinely used and often less extreme.

The objections work backwards:Denying the idealization would block the indeterminism.So deny the idealization.

Point mass of zero extension slides frictionlessly.Curvature singularity at the apex.Mass is initially located exactly at apex exactly at rest.Dome surface is perfectly rigid.

Dome: curvature

diverges, but tangent

everywhere defined.

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Two Types of Inadmissible Idealization

ExternalIdealization fails to represent some independently described specified system.

InternalIdealization relates badly to theory.

Example:The dome cannot represent any real dome due to atomic and quantum limits.

Examples:Logical contradiction.

Perfectly rigid bodies in theories prohibiting v>c.

Failure of standard methods.Point mass on surface that is continuous but nowhere differentiable.

Inapplicable.The dome is not an independently descibed system.

Inapplicable.Idealization of the dome no worse than elsewhere. Unless…

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?Badly Behaved Limits are Inadmissible Idealization?Admissible idealization must be like more realistic systems. If they are failed limits of more realistic systems, they cannot represent those more realistic systems well.

Trajectory of mass over rounded table edge unaffected by taking limit of sharp edge.

Admissible idealization.

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?Badly Behaved Limits are Inadmissible Idealization?

In the sequence of systems that approaches the limiting system:

property of the limiting system of the sequence

= limit of the properties of the systems in the sequence

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?Badly Behaved Limits are Inadmissible Idealization?Admissible idealization must be like more realistic systems. If they are failed limits of more realistic systems, they cannot represent those more realistic systems well.

Determinism of motions on dome with everywhere finite curvature changes discontinuously as we take limit of diverging curvature.

Inadmissible

idealization??

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?Badly Behaved Limits are Inadmissible Idealization?A discontinuity of properties at the limit just means that you have a different system.

NO

What is the lift generated by an infinite, non-rotating rotor?

Double rotor size.Halve rotor speed.

Rotor lift stays the same (at L).Power consumed is halved.

In the limit of infinitely many doublings,an infinite, non-rotating rotor generates lift L and consumes no power.

Is an infinite rotor at rest an inadmissible idealization since it generates no lift?!

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Forces in a perfectly rigid beam…

NO

…are statically indeterminate. Balance of all forces and moments:VA= VB/2= VC

butHA=HC = ANYTHING

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Forces in a beam with any elasticity, no matter how small…

NO

…are NOT statically indeterminate. Balance of all forces and moments fixes all individual forces.

Perfectly rigid beams are admissible Newtonian idealizations.

BUTConditions that can determine the forces in elastic beams are not strong enough to determine the forces in perfectly rigid beams.

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3. What’s New?

4. What’s Wrong?





6. Conclusion

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Newtonian systems with Finitely Many Degrees of Freedom

Generally deterministic.

Dynamical state fixed by n real valued variablesx1, x2, … , xn

dx1/dt = f1(x1, … , xn)dx2/dt = f2(x1, … , xn)

…

dxn/dt = fn(x1, … , xn)

Generally?Exceptions: simplest is the dome.

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Newtonian Systems with Infinitely Many Degrees of Freedom

Generally INdeterministic.when systems are sufficiently coupled so they don’t decompose into infinitely many systems each with finitely many degree of freedom.

Dynamical state fixed by infinitely many real valued variablesx1, x2, …

dx1/dt = f1(x1, …)dx2/dt = f2(x1, …)

…

Examples: supertasks with infinitely many masses, colliding, interacting via springs.

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Generate multiple solutions…

Choose system such that component n couples with components n-1, n+1.

dx1/dt = f1(x1, x2)

dx2/dt = f2(x1, x2, x3)

dx3/dt = f2(x2, x3, x4)

…

Up to minor distractions, this is the structure of masses and springs:

Fine printfn(xn-1, xn, xn+1) must be invertible in final argument and the inverse arbitrarily differentiable.x1(t) and x*

1(t) must be arbitrarily differentiable and agree in all derivatives at t=0.

Generate two solutions x1(t), x*1(t)

that agree for t<0 and at t=0 and disagree afterwards by…

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… by Iteration

dx1/dt = f1(x1, x2)

dx2/dt = f2(x1, x2, x3)

dx3/dt = f2(x2, x3, x4)

…

dx1/dt = f1(x1, x2)

dx2/dt = f2(x1, x2, x3)

dx3/dt = f2(x2, x3, x4)

…

Fix x1(t).

Solve for x2(t)

Know x1(t), x2(t).

Solve for x3(t)

Know x2(t), x3(t).

Solve for x4(t)

etc.

Similarly generate x*

1(t).

Indeterminism is generic since the iteration method is robust under small changes in the system.

dx1/dt = f1(x1, x2)

dx2/dt = f2(x1, x2, x3)

dx3/dt = f2(x2, x3, x4)

…

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6. Conclusion1. The Indeterminism


3. What’s New?

4. What’s Wrong?





6. Conclusion

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Conclusion

The dome provides the simplest example yet of indeterminism within Newtonian theory.No need for infinitely many interacting components, space invaders with unbounded speed.

It raises the questions:

What is Newtonian theory?

What grounds judgments that a system is “unphysical”?

When are idealizations inadmissible?

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